1. Field of the Invention
This invention relates to an image processing apparatus, an imaging system, and an image processing system, and in particular to a technique for assisting observation of an object with the use of a digital image.
2. Description of the Related Art
Recently, a virtual slide system attracts attention in the field of pathology, as a successor to an optical microscope which is currently used as a tool for pathological diagnosis. The virtual slide system enables pathological diagnosis to be performed on a display by imaging a specimen (a sample) to be observed placed on a slide and digitizing the image. The digitization of pathological diagnosis images with the virtual slide system makes it possible to handle conventional optical microscope images of specimens as digital data. It is expected this will bring about various merits, such as more rapid remote diagnosis, provision of information to patients through digital images, sharing of data of rare cases, and more efficient education and training.
When using a virtual slide system, it is required to digitize an entire image of a specimen to be observed placed on a slide in order to realize equivalent performance to that of an optical microscope. The digitization of the entire image of the specimen makes it possible to examine the digital data generated with the virtual slide system by using viewer software running or a PC or work station. The digitized entire image of the specimen will generally constitute an enormous amount of data, from several hundred million pixels to several billion pixels when represented by the number of pixels.
Even though the amount of data generated by the virtual slide system is enormous, this makes it possible to examine the specimen image either microscopically (in enlarged detail views) or macroscopically (in overall perspective views) by scaling the image with the viewer, which provides various advantages and conveniences. All the necessary information can be preliminarily acquired so that images of any resolution and any magnification can be displayed instantaneously as requested by a user. Further, by performing image analysis on the obtained digital data in order to comprehend a cell shape, calculate a cell count, or calculate an area ratio (an N/C ratio) between cytoplasm and a nucleus, for example, various information useful for pathological diagnosis can also be provided.
Incidentally, an imaging optical system of a virtual slide system is designed with an emphasis on resolving power and therefore has an extremely shallow depth of field. Accordingly, a range in which focus is achieved relative to the thickness of a specimen serving as an object to be imaged is extremely narrow, and therefore images of tissues and cells positioned away from a focal position in a depth direction (a direction along an optical axis of the imaging optical system or a direction perpendicular to an observation surface of a slide) are blurred. It is therefore difficult to observe an entire specimen from a single two-dimensional image. Further, in an image including a large amount of blur, the precision of characteristic amount extraction and image recognition decreases, leading to a reduction in the reliability of image analysis performed by a computer.
An image processing method known as focus stacking is available as a method of solving this problem. Focus stacking is a method of generating an image having a deep depth of field from a plurality of images obtained by imaging an object in various focal positions. Japanese Patent Application Publication No. 2005-037902, for example, discloses a system in which a deep-focus image is generated by dividing images having different focal positions respectively into a plurality of sections and performing focus stacking in each section.
According to the method disclosed in Japanese Patent Application Publication No. 2005-037902, an image that is in focus as a whole and includes little blur can be obtained. However, although this type of deep-focus image is useful for rough observation of the specimen as a whole, it is not suitable for detailed observation of a part of the specimen or comprehension of a three-dimensional structure and a three-dimensional distribution of tissues, cells, and so on. The reason for this is that when focus stacking is performed, depth direction information is lost, and therefore a user cannot determine front-rear relationships between respective structures (cells, nuclei, and so on) in the image simply by viewing the image. Further, when structures originally existing in different depth direction positions are overlapped on the image at an identical contrast, it is difficult to separate and identify the structures not merely through visual observation but even through image analysis using a computer.